Smad4 mediates transcriptional induction of AP1 family members in response to TGFβ. Total RNA from Smad4-deficient and Smad4-reexpressing SW480 cells and Smad4-positive LT97 colorectal adenoma cells was hybridized with specific probes for AP1 family members. Results were confirmed twice with similar results. Expression levels remained unaltered by the addition of recombinant TGFβ in Smad4-deficient SW480 cells. In contrast, all of these genes except for c-fos displayed increased mRNA levels in Smad4-reexpressing SW480 cells in response to TGFβ. Expression of c-fos appeared to be downregulated in Smad4-positive SW480 cells but strongly upregulated in LT97 cells.